Could some one please tell me if the glow plug temperature ratings have any bearing on hot your engine will run?

Its just I am trying to get the temperature down on my Genesis by lichenin up the fuel mixture. The lowest I can get it is ~140C (~284F).
If I go 1/4 turn more the thing won't pull away and stalls, which everything I have read says it is to rich!![]

I am in the UK and the temperature is currently around 15-20C (59-70F). Any recommendations on plug rating for this ambient temperature?

I am measuring the temperature with a MIP on board temperature gauge, and these are the peak temperatures it is reading.
All literature relates to driving the truck back to yourself and manually measuring it, but I have found it has lost 20+C (70F) by then.
So is a possibly momentary peak of ~140C (~284F) okay???